The invention relates to a microtome, in particular an ultramicrotome and specifically the seal which prevents escape of the cryogen contained inside the freezing chamber through the opening of the freezing chamber wall where the specimen carrier arm is guided through.
For cryogenic preparation of samples, in particular biological samples which are subject of microscopic or electron microscopic examinations after preparation, it is necessary to maintain both sample and knife at a low temperature during preparation. Cryogenic sample preparation is typically therefore performed in a microtome or ultramicrotome attached freezing chamber which typically is accessible from the top and in which a desired low temperature is provided by introduction of a gaseous cryogen. In order to hold the temperature consistent and to avoid penetration of damp air from above into the freezing chamber, it is necessary to continuously introduce cryogen.
For the production of thin and ultra-thin slices at low temperature a tool such as a knife is commonly attached to the bottom of the freezing chamber and the specimen to be cut is attached to a carrier arm, which is moveable upwards and downwards relative to the stationary knife. To obtain precision cuts, it is necessary to have at least two requirements:
a) the carrier arm must have a very rigid configuration; and PA1 b) the movement of the carrier arm during the cutting procedure cannot influence the atmosphere of the freezing chamber, in particular by allowing large amounts of cryogen to escape and causing a change of temperature inside the freezing chamber.
Unfortunately, it is difficult to design a carrier arm capable of holding and maneuvering a specimen inside the freezing chamber, which extends and is driven outside of the freezing chamber, and that complies with the above requirements.
There have been several attempts to solve this problem. For example, it is known to design the carrier arm like a bridge so that the arm reaches in an U-shape over the edge of the freezing chamber and the specimen is held at the low end of the U of the carrier arm inside the freezing chamber (so-called Christensen Bridge; U.S. Pat. No. 3,680,420). This configuration has the disadvantage that the arm, because of its U-shape, tends to lose its original shape, and has the tendency, particularly during operations involving increased cutting force, to deform, resulting in decreased cut quality. A similar design was disclosed in co-pending U.S. patent application No. 07/523,054.
According to another solution, the carrier arm is introduced through an opening into the chamber. The opening is excessively large to allow for the up and down movement of the carrier arm necessary for the cutting process. To avoid escape of cryogen, the edge of the opening is sealed to the carrier arm by means of a very thin and flexible membrane (DE-PS 1,622,996). The disadvantage of this design is the fact that the cold membrane exerts unwanted forces on the carrier arm, which has a disturbing influence on the up and down movement of the carrier arm and thus negatively influences the quality of the cut.
In another known design which introduces the carrier arm through an opening in one of the freezing chamber walls, the opening is configured as a "maze" where a number of disc shaped members located on the carrier arm extend into separate chambers for each membrane (U.S. Pat. No. 4,918,941). The membranes are moveable in the direction of the carrier arm. The membranes are arranged at the outside of the end of the carrier arm and moveable in its direction. Contact of the membranes with the respective chamber wall cause unwanted forces to occur during vertical movement of the carrier arm, resulting in vibration of the carrier arm and therefore negatively influencing the quality of the cut.
One object of the present invention is to provide a seal for the carrier arm of a microtome or ultramicrotome, which does not adversely influence movement of the carrier arm, yet at the same time prevents excess escape of cryogen, thereby maintaining the chamber atmosphere.